The present invention relates to a sensor and a device and method for a sensor self-test. Furthermore, the invention relates to an acceleration sensor.
In general, sensors are used in systems to detect measured quantities. In the field of inertial sensors, angular rate sensors and acceleration sensors are, for example, used to determine quantities of motion. Since these applications usually influence safety, the sensors must be extremely reliable and securely determine measured quantities.
German Published Patent Application No. 195 28 961 describes an angular rate sensor based on the tuning fork principle that is made of silicon. During operation, the tuning forks are excited to vibrate, and a sensor element registers the torsion of the tuning fork suspension that arises when the sensor rotates around an axis parallel to the tuning fork suspension.
To recognize sensor disturbance or drift, the sensor must undergo tests during operation. This increases the reliability and precision of the determined measured quantities.
European Published Patent Application No. 0708 925 describes a device to recognize errors in an impact sensor system in which a test reaction is generated when a sensor element is actuated, and the results are compared with the results of an expected reaction. U.S. Pat. No. 5,060,504 describes a method for the self-calibration of an acceleration sensor where a sensor mass is moved relative to a frame, and the movement corresponds to known acceleration. The initial value is used as a reference value for subsequent calibration. In the self-testing acceleration sensor described in U.S. Pat. No. 5,103,667, a mass is moved in a specific manner to test or calibrate the sensor, and the movement is measured.
These systems have the disadvantage, however, that the actual measuring must be interrupted during the sensor test. Measuring and testing alternate, and the measuring signal is not continuous over time. Furthermore, only a specific, predefined measured quantity is generated for the comparison, i.e., the overall measuring range of the sensor is not tested.
Acceleration sensors are, for example, used in vehicles to control the chassis and for navigational systems, handling dynamic systems, and passive safety systems (air bags). The continuously increasing number of sensors makes reliability a maximum priority. In the future, an increasing number of systems will actively influence driving, and rigid mechanical couplings will be replaced by electronic signals (drive-by-wire). A suitable signal control or the ability of sensors to test themselves is essential. Furthermore, acceleration sensors are being increasingly used in navigation systems for civil and military air travel.
Particularly in automobiles, capacitive acceleration sensors are frequently used as, for example, described by C. Lemaire and B. Sulouff in the article: xe2x80x9cSurface Micromachined Sensors for Vehicle Navigation Systems in Advanced Microsystems for Automotive Applications (D. E. Ricken and W. Gessner, editors, Springer, Berlin 1998, p. 103-112). These systems can also use the electrodes for capacitive readout to displace mass. This can be done at discrete intervals, but not continuously.
U.S. Pat. No. 5,834,646 describes a resonant acceleration sensor that essentially consists of a plate clamped several times. The plate serves as a resonator whose resonance frequency is disturbed from external acceleration, as well as a seismic mass. This arrangement can test the integrity of the mass/spring system, but cannot simulate the effect of acceleration.
Another resonant acceleration sensor is described in German Published Patent Application No. 198 12 773. It has a resonator structure that is excited to vibrate by initial electrical signals, and it emits secondary electrical signals depending on the measured quantity.
A permanent self-test has not been possible for the abovecited capacitive sensors as well. It is, however, possible to use additional structures such as additional capacitor combs to provide excitation, but they would require more space and cost more. Conventional capacitive sensors can therefore at most only run a static self-test at discrete intervals.
There is a need for a sensor, especially a rate of rotation or acceleration sensor with a precise, resonant signal evaluation that can run a permanent or ongoing self-test.
It is therefore one object of the present invention to provide a sensor and a device and method for a sensor self-test in which measurement is uninterrupted during the test and the measuring signal is not impaired, and the self-test can run continuously during measuring.
The above and other beneficial objects of the present invention are achieved by providing a sensor, a sensor self-testing device, a method for sensor self-testing and an acceleration sensor as described herein.
The sensor according to the invention includes a resonant structure to detect a measured quantity, an actor unit to excite the structure to cause an initial, periodic oscillation, an element to generate an output signal that depends on the measured quantity, and one of a detector and an isolator respectively configured to detect and isolate the test signal from the output signal that is generated by a second periodic vibration of the structure that is superposed on the first vibration. The sensor can test itself and is able to simultaneously provide a continuous measuring signal and a test signal that provides information on the functioning of the sensor. The measurement is not interrupted, and the measuring signal is not impaired.
The sensor may also include elements configured to generate the second periodic vibration of the structure, and the first and second periodic vibration may be generated, e.g., by the same actor elements. The second vibration may also be generated by mechanical crosstalk. In operation, the vibrations of the structure are detected to generate the output signal.
According to another aspect of the present invention, a device is provided for sensor self-testing, and the sensor determines a measured quantity via a resonant structure and generates a periodic output signal depending on the measured quantity. The self-testing device includes an isolator configured to isolate a test signal component superposed on a useful signal component from the periodic output signal of the sensor and may also include a comparator configured to compare the test signal component with a predefined value or with a test signal fed to the sensor. A sensor self-test may be performed with a device that is continuous and may cover the entire measuring and dynamic range of the sensor without influencing the actual measuring.
The sensor or device may include a device configured to modulate a test signal on a signal for exciting the resonant structure. This arrangement allows the self-test to be adjusted over the entire measuring range of the sensor.
The sensor may include a mechanical unbalance or crosstalk that generates the test signal component which is used to test the sensor. This arrangement reduces the number of components and permits economical manufacture. The sensor may include an actor element configured to generate the second vibration that causes the test signal component, and the device may include an exciting device configured to excite the structure to a second vibration mode that is superposed on a first vibration mode which serves to detect the measured quantity. This arrangement allows specific test signals to be modulated onto the useful signal, and the sensor output signal may be evaluated according to the measured quantity and response of the system to the test signal.
The output signal may be analyzed by a device configured for frequency and/or phase analysis. The test may cover the entire measuring and/or dynamic range of the sensor using an arrangement configured to periodically change the amplitude and/or frequency of the test signal. In particular, the sensor may be an angular rate sensor, acceleration sensor or pressure sensor.
The sensor according to the invention may include, e.g., a resonator as the resonant structure and a linked resonant mass that changes that the resonance frequency of the resonator when it is displaced. The mass may be caused to vibrate during measurement to generate the test signal component.
The test signal component may be used to calibrate the sensor.
During operation, the actor unit, e.g., may vibrate a resonator and a linked mass in different modes, whereby the resonant mass periodically changes the resonance frequency of the resonator to generate the test signal component.
The sensor, e.g., may include a demodulator configured to demodulate the measuring signal, whereby the signal may be amplitude modulated or frequency modulated.
The method according to the invention for self-testing a sensor with a resonant structure includes the steps of: superposing second periodic vibration on a first vibration of the structure that is used to detect a measured quantity; determining an output signal that contains information on the measured quantity linked to the resonant structure; and monitoring a test signal component contained in the output signal that is generated by the second periodic vibration of the structure.
The amplitude of an excitation signal may be modulated by a test signal to generate the first vibration of the structure, and the frequency and/or amplitude of the test signal may be varied while the sensor is measured. In the method according to the invention, the entire functioning of the sensor including its electronics may be verified over its entire dynamic range. The method according to the present invention greatly increases the intrinsic safety of the sensor, which is particularly important for applications that influence safety.
The frequency of the excitation signal may also be modulated by a test signal to perform the self-test during measurement.
The test signal component in the output signal may be used to calibrate the sensor. The structure may be excited to vibrate at a minimum of two frequencies or modes. The first frequency or mode may represent the quantity to be measured, and a second frequency or mode may represent a test signal.
The acceleration sensor according to the present invention includes a resonator coupled to a mass so that its resonance frequency changes when the mass is displaced, an actor unit configured to excite the resonator, a detector configured to generate an output signal that depends on the resonance frequency, a control unit configured to generate an excitation signal that includes at least two frequencies to simultaneously excite the resonator and mass, and an evaluation step that separates from the output signal the vibration mode of the mass as a test signal component.
The sensor according to the present invention allows all relevant components of the evaluation electronics to be tested during the self-test. Recalibration may also be performed at set intervals.